MonoidK instances for Map and SortedMap

[alexeygorobets]

solve #2714

[codecov-io]

Codecov Report

Merging #2744 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2744      +/-   ##
==========================================
+ Coverage    95.3%   95.33%   +0.03%     
==========================================
  Files         363      363              
  Lines        6771     6777       +6     
  Branches      291      299       +8     
==========================================
+ Hits         6453     6461       +8     
+ Misses        318      316       -2

Impacted Files	Coverage Δ
testkit/src/main/scala/cats/tests/CatsSuite.scala	33.33% <ø> (ø)	⬆️
core/src/main/scala/cats/instances/map.scala	95.12% <100%> (+0.38%)	⬆️
core/src/main/scala/cats/instances/sortedMap.scala	97.5% <100%> (+0.09%)	⬆️
...rc/main/scala/cats/kernel/instances/function.scala	100% <0%> (+3.03%)	⬆️
testkit/src/main/scala/cats/tests/Helpers.scala	100% <0%> (+4%)	⬆️

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[LukaJCB]

Thanks, great addition! :)

[kailuowang]

Looks great to me. Thanks a lot!